Because of their superior magnetic properties, silicon sheet steels are widely used in the production of magnetic core components in electrical equipment such as motors, generators, transformers, and the like. These favorable magnetic properties, namely high magnetic permeability, high electrical resistance and low hysteresis losses, will minimize wasteful conversion of electrical energy into heat, and will therefore permit manufacture of electrical equipment having greater power and efficiency. In order to effect and optimize the desired magnetic properties, however, the silicon sheet steels must be produced under carefully controlled and exacting processing parameters. Silicon sheet steels are therefore substantially more expensive than other more conventional flat rolled steel products.
In the high volume manufacture of small electrical equipment for consumer appliances, toys and the like, unit cost is perhaps the most important consideration, far outweighing equipment efficiency and power considerations. For these applications, therefore, electrical equipment manufacturers frequently utilize the less expensive, more conventional low-carbon sheet steels for magnetic core components. Hence, there is a considerable market for low-carbon sheet steels having acceptable magnetic properties for magnetic core applications.
In the course of producing low-carbon sheet steels for magnetic applications, economic considerations have dictated that expensive processing steps be avoided and that even inexpensive steps be minimized. Therefore, even though elaborate processes have been developed for producing low-carbon sheet steels having exceptional magentic properties, such processes have not been adapted commercially, because the use of such processes would greatly add to the cost of the product, while not improving the magnetic properties of the resultant sheet sufficiently to equal those of silicon sheet steels having comparable cost of production. To be of any commercial value, therefore, any new process for improving the magnetic properties of low-carbon sheet steels must be one that will not significantly increase the steel's production cost. Commercially, therefore, low-carbon sheet steels for magnetic applications are produced from conventional low-carbon steel heats having less than 0.1 percent carbon and the usual residual elements at normal levels for cold-rolled products. The rolling procedures are similar to those used for other cold-rolled products. Specifically, the production steps are usually limited to hot rolling a low-carbon ingot to slab form; hot rolling the slab to sheet form; pickling the hot rolled sheet, cold rolling the pickled sheet for a reduction of 40 to 80 percent, and annealing the sheet to effect recrystallization, generally in a box annealing furnace. An optional final temper roll of from 1/2 to 2 percent is sometimes provided for the purpose of flattening the resultant sheet and make it better suited for subsequent slitting and punching operations. Alternately, more recent developments have shown that temper rolling from 7 to 9% will not only impart the desired flatness and punchability characteristics, but will also improve the magnetic properties, as disclosed in U.S. Pat. No. 3,923,560.
The commercially produced low-carbon sheet steels for magnetic applications, when rolled to 18.5 mils thickness, typically exhibit permeabilities in the rolled direction of from 5000 to 6000 at 10 kilogauss, with core losses of from 1.3 to 1.6 watts/lb. For the same thickness at 15 kilogauss, permeabilities in the rolled direction typically range from 2000 to 4000 with core losses of 3.0 to 4.0 watts/lb. Sheets rolled to 25 mils typically exhibit permeabilities in the rolled direction of from 4200 to 4800, with core losses of 1.8 to 2.0 watts/lb. and 10 kilogauss; and permeabilities in the rolled direction of from 2000 to 3000 with core losses of 4.2 to 4.8 watts/lb. at 15 kilogauss.
These relatively wide ranges in magnetic properties reflect an established tendency on the part of industry to deemphasize magnetic properties in low-carbon sheet steel and emphasize low cost of production. Nevertheless, customers have recently begun to demand improved magnetic properties, particularly at 15 kilogauss, without an appreciable increase in cost. As noted above, producers have been hard pressed to improve magnetic properties in these steels without substantial increases in production costs.
Recently developed low-carbon sheet steels have shown marked improvement in core loss values. Specifically, exceptionally low-carbon steels, i.e. 0.01 to 0.02% carbon, having manganese and phosphorus contents of about 0.5 to 1% and 0.12 to 0.18% respectively, can be processed to produce 15 kg core loss values of 2.3 to 2.7 watts per pound when rolled to 18.5 mils thickness. Unfortunately, however, the 15 kg permeabilities for these steels, typically within the range 1800 to 2000, are not superior to many of the low-carbon electrical sheet steel available prior thereto. Although these newer steels have achieved a considerable degree of commercial success, based on their superior core loss characteristics, they have not been acceptable in those applications wherein good permeability is also essential or of prime consideration.